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Fan Motor

Discussion in 'Electronic Basics' started by [email protected], Apr 21, 2008.

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  1. Guest

    Sorry for such a simple question, but I've got a few 120mm
    240V .125A (?30W) axial fans like a bigger version of the case
    cooling fans in a computer.

    I've set one up in a box with a large truck air filter so I can run it
    24/7 to remove dust from my house atmosphere. At the moment, the
    aircons and computers do it :)

    Unfortunately these fans are a little too energy hungry and noisy.

    What are my options for slowing them down a tad?

    I thought of adding a resistance in line, but this would only
    dissipate heat -- not lost in Winter, but a waste of money in Summer.

    Then I thought about running two of these in series.

    Is there any cheap and cheerful way to cut down on these over
    enthusiastic, but otherwise great little ball bearing fans?

    Thanks, jack
  2. Rich Webb

    Rich Webb Guest

    PWM is the usual way. Some good discussion at
  3. neon


    Oct 21, 2006
    TAD it means to me English Irish Well nevertheless there are TRIACS that will conduct only during a portion of the phase cost to buy USA $20 to make it is simple but it may not look good but will work.simply look for TRIACS design and i guess copy and inplement. the power loss will be minimumn maybe 1 watt from 100% to %50. on the internet there are these circuits availavleto copy from
  4. Guest

    Thanks guys. I'll look for a fan speed controller, but I guess they
    usually only come cheap with fans?

    I've read that a transformer would be OK.
    I see a cheapie 36V that would suit.
    What speed would the fan likely do (fraction of full speed) at 36V?
    Perhaps around half speed, if the curve I/rpm is logarithmic?

    Then someone suggested a diode in one lead. Is that liable to damage
    the motor? Perhaps a tranny is safer.

    I don't really need a variable speed, just roughly halve the full

    Cheers, jack
  5. Guest

    Could someone check my calculation, please?

    The motor draws 125mA at 240V.
    So at 36V, it should draw roughly 36/240 x 125 = 18.75 mA
    The VA (~Watts) here will be 18.75 x 36/1000 = 0.675 W
    Which will cost me ~ $1 per annum to run.

    What would be the approximate efficiency of the transformer?

    Cheers, jack
  6. amdx

    amdx Guest

    At some point the voltage will be to low to start the motor, it will run
    if pushed, it just won't start.
    My favorite solution is to use a series capacitor (non polarized, voltage
    appropriate) to drop some of the voltage. The capacitor is smaller than a
    transformer and produces much less heat than a resistor.
    I don't have the math ability to calculate the proper capacitance and you
    don't know what voltage you want on the fan. So use a transformer, resistor,
    or variac to find the proper speed then measure the voltage across the
    motor. With the wanted voltage known, someone on here can calculate the
    proper capacitance.
    I would try putting the two fans in series that may be the simplest
    Anyone know how to model that motor in LTspice. Is it as simple as an
    inductor? (or as complicated :))

  7. neon


    Oct 21, 2006
    after you buy the transformer for penneys there is going to be STIL losses as heat that is not cheap
  8. neon


    Oct 21, 2006
    first of all a diode will give you pulseting DC. a transformer is ok but still there are losses to contend with. transformers AC type are 20% efficient and they are fixed in output. A dimmer is the way to go. make your own for $3 or buy for $20 or get a life.
  9. Rich Grise

    Rich Grise Guest

    It depends on what kind of motor. Different types use different
    methods of controlling the speed. Most of them don't like just
    reducing the voltage to them. Do you have any more information
    about the motor, like a brand name, part number, maybe a nameplate
    with some information?

  10. Guest

    Thanks for your help Rich.

    The fan is a Sirocco brand.
    Model YX2517
    Ball bearing
    AC230/240V 50Hz
    125 mA
    Impedance Protected
    Made in Taiwan

    That was from the label on the fan.

    The cattledog says:
    Air volume 3 m^3/min
    rpm 3000
    Input Watts 17
    Current 125 mA
    Noise 47 dBA

    Maybe a diagnostic feature is that it freewheels without any cogging
    for a long time after you switch it off. I guess that means there are
    no permanent magnets in it.

    I realise that it would be the ideal to buy another fan with less
    current draw, but these are what I've got, and I can't see anything
    more suitable. These were spot on, on paper, just that the acid test
    shows them to be noisy little buggers -- I should have investigated
    that 47 dB :)

    Cheers, jack
  11. Rich Grise

    Rich Grise Guest

    Well, it sounds like it's definitely probably maybe an induction motor.

    Controlling the speed of an induction motor, short of a VFD, is kind of
    out of my league - maybe try a google (or any handy search engine) search
    on "control induction motor speed" or the like.

    Good Luck!
  12. Guest

    Thanks, Rich, I've done some reading and it seems that small fans on
    mains power are generally large-slip motors which are amenable to
    speed control with voltage, as opposed to most induction motors that
    approach synchronous speeds and not much away from that.

    Now, I calculated that this fan has an "effective resistance" of
    around 3.3 kOhms. I happen to have a 3.3 kOhm 10W resistor handy and
    so I connected it in series with this fan.

    The fan ran magnificently. Just about right, maybe a little faster
    would be better, but perfectly quiet and acceptable.

    Unfortunately, after about 3 minutes, the resistor was so hot it could
    have burned me. I know that wirewound cement resistors can run really
    hot, but this appeared to be absorbing more Watts than my calculation
    would have led me to believe.

    The motor says it draws 0.125 A but the catalogue says "input Watts
    17" So I assume that that current is startup current. And the running
    current is actually 17/240 = 0.071 A.

    I suppose the effective resistance (reactance, impedance and all that)
    of the motor can only be found by measurement, so it's possible that
    the resistor was absorbing anything up to about 15W in the circuit
    that I tried it.

    Anyways, that is going to be too expensive to run 24/7.
    I can't seem to find a transformer that would drop 240VAC to around
    150VAC or so. I have a book of calculations for making your own
    transformers, is this an efficient possibility?

    Perhaps stripping out the secondary of an existing tranny and rewiring
    to suit? Are big transformers less efficient than one just above
    specs? Do you know of an easier way to efficiently drop the mains
    voltage to two thirds?

    Disclaimer : All advice taken without any implied responsibility of
    the adviser. Rest assured that I treat electricity with all the
    paranoia it deserves. No touchy when plugged in!

    Cheers, and thanks, jack
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